How to copy weights from one bone to another?
Nevermind, I'm silly. 

The way to copy bone weights to other bones is by making a copy of a vertex group that belongs to the bone you wish to mirror. Let's say "Hand.L" and then clicking on the black arrow in the vertex group panel and selecting "Mirror Vertex Group", then change the name to "Hand.R" and if you now move that bone that is "Hand.R" it will deform the mesh just like on the left side.